1. Field of the Invention
This invention relates to downhole apparatus for determining the properties of subsurface formations. More particularly, the invention relates to methods and apparatus for detecting subsurface radiation phenomena.
2. Description of Related Art
The characteristics of geological formations are of significant interest in the exploration for and production of subsurface water and mineral deposits, such as oil and gas. Many characteristics, such as the hydrocarbon volume, porosity, lithology, and permeability of a formation, may be deduced from certain measurable quantities. Among these quantities are: density, neutron porosity, photoelectric factor (Pe), hydrogen index, salinity, and thermal neutron capture cross section (Sigma). The quantities are typically measured by logging-while-drilling (LWD) and wireline tools. A typical tool carries a source that radiates/emits energy into the formation and one or more detectors that sense the resulting interaction of the radiation. Detected signal data is typically transmitted uphole, temporarily stored downhole for later processing, or combined in both techniques, to evaluate the geological formation from which the data was gathered.
Of the many quantities of interest in exploration for and production of hydrocarbons, the density and photoelectric factor of the formations are two of the most important. These quantities are generally measured with a tool containing a source of gamma rays and at least one gamma-ray detector (See, e.g., U.S. Pat. Nos. 5,390,115, 5,596,142, 6,376,838, 5,528,029, 4,691,102). The depth of investigation of this measurement is relatively shallow, on the order of a few centimeters. Consequently, the measurement is sensitive to the environment near the tool body. In particular, borehole fluid (mud) or mud cake interposed between the tool and the formation degrades the accuracy of the measurement.
To reduce these environmental effects, the source and gamma-ray detector(s) are typically heavily shielded and collimated. Additionally, two gamma-ray detectors are also commonly disposed in the tool. The detector further from the source is generally used to obtain the primary density measurement and the one nearer to the source a correction for near-tool effects. For radiation-type tools conveyed through the formation on a drill pipe, these techniques are essentially all that can be applied to improve the accuracy of the measurement.
However, an additional technique for minimizing the separation between tool and formation is available for tools conveyed through the formation via wireline, slickline, coiled tubing, tractors, or through drill pipe. This technique is shown in FIG. 1A. In it, a source 5 (e.g., a gamma-ray source) and one or more detectors 12 are place in a pad 14. This pad 14 is typically connected by a hinged joint to the support member or main body 16 of the tool 10 as known in the art. Mechanical and electrical support for the pad 14 is provided by the tool body, which in the region of the pad may be referred to as the “C-housing” recess 20 due to its cross sectional shape (See FIG. 2A). A biasing or back-up arm 18 is attached to the back of the pad 14 in order to force it away from the main body 16 and into contact with the formation 22.
As shown in FIGS. 1A and 1B, in the logging operation, the back-up arm 18 forces the exposed surface 24 of the pad 14 into contact with the borehole 26 wall. In FIG. 1A, the borehole 26 is smooth and the pad 14 is inside the recess 20. In FIG. 1B, the borehole 26 is washed-out and the pad 14 is extended from the recess 20.
This arrangement allows the source 5 and detector(s) 12 to remain close to the formation 22 under a variety of conditions. In a smooth borehole 26 with no mud cake (FIGS. 1A and 2A), the pad 14 is in contact with the formation 22 and is seated inside the recess 20. In washed-out or rugose boreholes 26 (FIGS. 1B and 2B), the pad 14 is still in contact with the formation 22, but it is now extended from the recess 20. Had the detector(s) 12 been encased in the tool body 16, there would be a considerable amount of borehole mud between the tool 10 and the formation 22 in this situation, potentially degrading the accuracy of the measurement.
While the use of a pad-based tool reduces the difficulty of maintaining good tool-formation contact in non-ideal situations, it presents a potential problem. Formation density and Pe are typically measured by monitoring the changes in the number and distribution of detected gamma rays under the assumption that these changes arise only from changes in the formation or mud properties or from standoff between the tool and borehole wall. Comparing FIGS. 2A and 2B, another effect that may cause a variation in the detected radiation is the position of the pad 14 relative to the recess 20. This position changes dynamically as the well is logged due not only to washouts and rugosity, but also to the precise articulation and method of conveyance of the tool and to the trajectory of the borehole. If the effect is large and uncorrected, it will introduce error into the measurement.
This error is likely larger when less-dense borehole fluids are involved. The radiation that interacts with the recess at some point during travel from the source 5 to the detector 12 gives rise to the sensitivity of the measurement to the recess 20 position. These gamma rays must necessarily pass through the borehole 26 to reach the recess 20. Greater attenuation is likely in boreholes filled with denser fluids compared to less dense fluids, and hence this recess effect will generally be larger in the latter case. In particular, operating pad-based radiation-sensitive measurement tools in air-filled boreholes may be especially prone to this problem.
Earlier generation nuclear-type logging tools have used massive amounts of shielding. The intent of this shielding has been to restrict the detected radiation (e.g. gamma rays) to that which travels mainly in the formation near the line of closest approach between the tool and the formation. The result is a measurement that is more focused and less sensitive to borehole diameter and borehole fluid.
These conventional shielding techniques work for their intended purpose, but with several drawbacks. In order to effectively attenuate undesired radiation, shielding materials must contain elements with high atomic numbers and high densities. Moreover, the energies of the radiation involved and the sensitivity of the measurement require that the shielding be very thick. These shielding materials are also difficult to form and to machine, and few vendors are willing to do so. These features combine to create tools which are large, heavy, and expensive.
Conventional downhole tools, on the other hand, are often expected to be small, light, and inexpensive. Under these conditions, the space available for shielding is much more restricted. The measurement is necessarily less focused and sensitivity of the measurement to recess position can be expected to occur unless additional steps are taken. Nuclear modeling calculations can confirm this. For example, for a pad in an air-filled borehole, the apparent density can change by ˜0.1 g/cm3 with the relative position of pad and recess, about ten times larger than a desired accuracy of ˜0.01 g/cm3.
Thus a need remains for improved shielding techniques in radiation-type tools in order to reduce undesired effects on the measurements.